pgsql: Use PQExpBuffer for error message in fe-auth.c.

Log Message:
-----------
Use PQExpBuffer for error message in fe-auth.c.
In passing, change functions that passedin both PGconn and
parts of it to just pass in the PGconn.

Modified Files:
--------------
    pgsql/src/interfaces/libpq:
        fe-auth.c (r1.129 -> r1.130)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.129&r2=1.130)
        fe-auth.h (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.h.diff?r1=1.26&r2=1.27)
        fe-connect.c (r1.350 -> r1.351)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c.diff?r1=1.350&r2=1.351)
        libpq-int.h (r1.124 -> r1.125)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-int.h.diff?r1=1.124&r2=1.125)
        pqexpbuffer.h (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/pqexpbuffer.h.diff?r1=1.17&r2=1.18)